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ADG794 Synchronously Rectified Charge Injection

Hello.

I'm trying to synchronously rectify and integrate a high-impedance AC signal using an ADG794 switch in the circuit shown below.

It works for lower values of R1, but once the current drops below about 100nA RMS, the integrator goes the wrong way (i.e., the rectified current is in the opposite direction of what it should be).

The signal frequency (V1) is much higher than the R1 C1 time constant, so the phase shift should be negligible. I've tried offsetting the switch and signal frequency clocks in time. The system is effectively dual supply and nothing exceeds the ADG794 supply rails. The issue does not seem to be DC leakage, because the integrator holds a constant charge for a long time if the switch remains off. Using a low input-offset op amp does not seem to improve anything. The only thing I've been able to come up with is that the switch's charge injection is also being synchronously rectified.

Does this make sense? Is there a better explanation I could test? Is there a model for this switch or a similar one that would show the problem? Is there a solution besides adding a current amplifier before the switch?

Thanks,

Dan

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  • Thanks for the prompt response.

    I am using 3.3 V rails for the switch, but the 'ground' shown in the diagram is at 500mV to be within the switch and op amp input range. The important thing is that nothing exceeds the switch rails.

    C1 is 100nF, C2 is 10nF, and V1 is a 80mV square wave at 100kHz. The circuit works fine when R1 is 100kOhms, but if I increase it to 2M (or disconnect it) it does not.

    I'm using an old analog scope, so I can't give you a good waveform, but here's what it's supposed to look like (e.g., what it looks like when R1 is 100kOhms).

    The switch is in sync with the excitation (V1 in the schematic). The issue is that the integrator output reverses direction (integrates down instead of up) when R1 is 2MOhms.

    Both your article and my previous understanding of charge injection tell me that the charge injection should not result in a DC current like the one I'm seeing, but I don't have any other explanation for what could be happening.

    Thanks,

    Dan

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  • Thanks for the prompt response.

    I am using 3.3 V rails for the switch, but the 'ground' shown in the diagram is at 500mV to be within the switch and op amp input range. The important thing is that nothing exceeds the switch rails.

    C1 is 100nF, C2 is 10nF, and V1 is a 80mV square wave at 100kHz. The circuit works fine when R1 is 100kOhms, but if I increase it to 2M (or disconnect it) it does not.

    I'm using an old analog scope, so I can't give you a good waveform, but here's what it's supposed to look like (e.g., what it looks like when R1 is 100kOhms).

    The switch is in sync with the excitation (V1 in the schematic). The issue is that the integrator output reverses direction (integrates down instead of up) when R1 is 2MOhms.

    Both your article and my previous understanding of charge injection tell me that the charge injection should not result in a DC current like the one I'm seeing, but I don't have any other explanation for what could be happening.

    Thanks,

    Dan

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